1. Field of the Invention
The present invention relates to a vacuum actuator that drives a mechanism portion in a vacuum atmosphere, and a substrate transport robot and, more particularly, to a vacuum actuator which serves as a driving source for a transport apparatus which transports, for example, a substrate in a vacuum atmosphere, and a substrate transport robot having the vacuum actuator.
2. Description of the Related Art
As an actuator which drives a mechanism portion in a vacuum atmosphere, an actuator, in which a rotor to which a permanent magnet is attached is disposed on the vacuum side and a stator which generates a magnetic field is disposed on the atmospheric side by disposing a partition wall between the rotor and the stator, has been known (see, for example, Japanese Utility Model Laid-Open Nos. 62-51943 and 64-16157 and Japanese Patent Laid-Open Nos. 2001-112223 and 05-316706).
A coil of the stator is formed by winding a conductive wire and therefore has a surface area corresponding to the length of the conductive wire. Any adverse effect that an out-gas discharged from the coil exerts on the vacuum side is prevented by providing the actuator with a partition wall which separates the coil from the vacuum side, as in the motors disclosed in Japanese Utility Model Laid-Open Nos. 62-51943 and 64-16157.
However, according to techniques presented in Japanese Utility Model Laid-Open Nos. 62-51943 and 64-16157, because the space which accommodates the rotor and that which accommodates the stator are separated from each other by the partition wall, the permanent magnet fixed on the rotor and the distal end of the stator have a relatively large distance between them. This inevitably keeps low the efficiency (torque constant) at which the torque can be produced in practice by the rotor for a current supplied to the coil. Although a lot of efforts have been made to decrease the thickness of the vacuum partition wall portion, a thickness which generates a rigidity high enough to withstand the atmospheric pressure is required, and gaps must be formed between the partition wall and the rotor and between the partition wall and the stator for interference prevention during operation and in terms of assembly, so no significant improvement can be achieved.
Similarly, because the space which accommodates the rotor and that which accommodates the stator are separated from each other by the partition wall, the rotor is positioned relative to the partition wall. On the other hand, the stator must inevitably be positioned relative to the partition wall as well. This makes it difficult to ensure a given accuracy of positioning the rotor relative to the stator, that is, a given coaxial accuracy between them, thus leading to disadvantages such as an increase in amount of vibration of the motor, deterioration in efficiency, or degradation in assembly performance.